The present invention relates to an optical pick-up apparatus which performs at least either reproduction or recording of information on a plurality of types of optical information recording media having respective different thickness of protective substrates each other, and an objective lens for the optical pick-up apparatus.
With a short wavelength type red color semiconductor laser which has recently been put to practical use, commercialized has been a high density DVD (digital versatile disc) that is the same in size as a conventional CD (compact disc) but has a larger capacity as an optical information recording medium. When information is recorded on this DVD or reproduced from this DVD, it is necessary to utilize a short wavelength type red color semiconductor laser having a wavelength of 650 nm and an objective lens having a numerical aperture (NA) of approximately 0.6.
Further when information is recorded on CD or reproduced from CD, the semiconductor laser of wavelength of 780 nm is utilized, separately from the short wavelength semiconductor laser for DVDs, and the numerical aperture NA of the objective lens requires an approximate value of 0.45.
Further, required is an optical pick-up apparatus, featuring interchangeability of information recording media so as to perform recording and/or reproduction of information for both the CD and DVD on one apparatus. Information recording surfaces of these optical information recording media are protected by transparent base boards (or protective substrates), and the thicknesses of which are different from each other, depending on the specifications of each optical information recording medium, and have been determined to be 0.6 mm for DVD, and 1.2 mm for CD, respectively. In order to realize interchangeability between DVD and CD, each having a different thickness of transparent base boards, it is necessary to correct for spherical aberration caused by the thickness of the base boards, and thereby, an objective lens using diffractive light is proposed (See TOKKAI 2000-81566, for example).
The spherical aberration varies due to temperature change in the optical system of the optical pick-up apparatus. That is, a change in the refractive index of a plastic lens, such as generally used in the optical systems, is largely dependent on temperature change, and the spherical aberration varies based on the change of the refractive index. The emission wavelength of a laser source also varies depending on temperature, which causes a change of the spherical aberration. Influence on the recording and reproduction of information caused by the change of spherical aberration, quickly becomes excessive for DVDs which use short wavelength laser rays and an objective lens featuring a high numerical aperture. Proposed has been an objective lens having the nature to depend on the wavelength, in order to correct the spherical aberration caused by the change of refractive index of an objective lens due to changes of temperature (See TOKKAI 2001-249273).
However, TOKKAI 2001-249273 proposes only a countermeasure for the objective lens, and does not consider the whole optical system. Accordingly, in an optical pick-up apparatus having interchangeability between optical information recording media such as between CD and DVD, both of which are different types, it is necessary to devise a countermeasure for the change of spherical aberration, caused by temperature change in the whole optical system, employing the short wavelength laser rays and an objective lens with a high numerical aperture.